Image-forming devices such as electrophotographic devices, electrostatic recording devices, electrostatic printing devices, and so on are applied to copying machines, printers, facsimile machines, complex machines thereof and so on. A method of forming a desired image by developing an electrostatic latent image formed on a photosensitive member with a toner for developing an electrostatic image is widely employed.
For example, generally, an electrophotographic device using electrophotography uniformly charges the surface of a photosensitive member generally formed of photoconductive material with any of the various means, and then, an electrostatic latent image is formed on the photosensitive member. Next, the electrostatic latent image is developed using a toner. After transferring an image of the toner on a recording material such as paper or the like if necessary, the image is fixed by heating or the like. Thus, a copy is obtained.
In recent years, as toners used for development, positively-chargeable toners applicable to the positive charging method are preferably used from the viewpoint of inhibiting ozone generation and obtaining the toners excellent in charge property.
In addition, generally, microparticles of an external additive are externally added (attached by addition) on the surface of the toner particles from the viewpoint of improving the charge property and flowability of toners.
However, in the process of continuous printing of plural images using a conventional external additive, microparticles of the external additive are buried on and/or released (detached) from the surface of toner particles due to mechanical stress in a development device, such as increase in number of contact of toner particles by agitation or the like, and thus decrease functions thereof as external additives. Thereby, conventional external additives are less likely to impart a stable charge property and flowability to toner particles over time.
The toner particles, the microparticles of the external additive being buried, and the microparticles of the external additive released (detached) from the surface of the toner particles may cause damage on photosensitive members, and filming due to the toner particles attaching to the surface of a photosensitive member. Thus, there are problems that deterioration of image quality due to fog, blur, white streaks or the like tends to occur, and printing durability of toners decreases upon continuous printing of plural prints.
With respect to the problems of deterioration of image quality due to the burial and release of the external additive, development of a toner is demanded, wherein the toner can maintain the state in which microparticles of an external additive are suitably attached to the surface of colored resin particles over time even if the number of contact of the toner particles by agitation or the like is increased in a development device in the process of continuous printing of plural prints.
Patent Literature 1 discloses a toner which can prevent damage of photosensitive members and filming by externally adding magnesium silicate, preferably forsterite, steatite or enstatite having a specific hardness (Mohs hardness) of 4.5 to 8, to the surface of toner particles as an external additive.
Patent Literature 2 discloses a toner which is formed by externally adding white filler particles, preferably talc, having a specific volume average particle diameter of 1 to 100 μm, to the surface of toner particles as an external additive, and thereby prevents damage of photosensitive members and soiling in non-image areas due to decrease in flowability and charge property of the toner particles.
In the toners disclosed in Patent Literatures 1 and 2, improvement of deterioration of image quality caused by burial and release of microparticles of an external additive has been attempted by defining the hardness and the particle diameter of the external additive. However, as a result of researches, the inventor of the present invention found out that the external additives used in Patent Literatures 1 and 2 do not necessarily have sufficient effect of preventing damage of photosensitive members and filming.
Patent Literature 1: Japanese patent Application Laid-open (JP-A) No. 2006-330689
Patent Literature 2: JP-A No. H8-339095